Developing　high　performance　n-type　thermoelectric　(TE)　materials　is　fundamentally　important　for　developing　high　efficiency　TE　devices.　AgBiSe2,　which　reveals　superior　n-type　TE　performance　in　a　cubic　phase,　crystallizes　in　a　hexagonal　phase　at　room　temperature,　and　typically,　undergoes　phase　transitions　to　a　cubic　phase　at　a　temperature　above　580　K.　Here,　for　the　first　time,　through　entropy　optimization　with　lead-selenides　(≥9.9　mol%),　the　high-temperature　cubic　phase　of　AgBiSe2　is　stabilized　from　300　to　800　K.　Furthermore,　the　AgBiSe2-PbSe　pseudo-binary　diagram　is　established.　The　resultant　alloys　with　optimized　entropy　possess　unique　local　distorted　cubic　lattices,　which　contribute　low　lattice　thermal　conductivity　approaching　0.3　W　m−1　K−1　in　extended　operating　temperature　range.　Consequently,　a　peak　figure　of　merit　zT　value　of　≈0.8　at　800　K　and　a　record-high　average　zT　value　of　0.42　for　n-type　I-V-VI2　compounds　are　attained　in　pure　phase　cubic　n-type　(AgBiSe2)1−x(PbSe)x　solid　solutions.　These　results　pave　the　way　for　developing　new　TE　materials　via　entropy　engineering.　©　2020　Wiley-VCH　GmbH